Predictors use AC "track circuts" (AFO of sorts) and computerized logic to measure the rate of change in inductance in the loop formed by the rails and leading wheels. As the wheels move closer to the crossing, the inductance diminishes, and this rate of drop is used to calculate train speed and turn on the crossing lights at just the right time no matter how fast or slow it's going. If the wheels stop moving toward the crossing for a predetermined amount of time (i.e. the change in inductance stops or starts increasing because the wheels have started moving away from the crossing), the crossing portection is shut off. There are also presence detectors, which function just like the loops in the street for traffic signals. The inductance of the loop is changed by the presence of large metal objects over them, so whenever the section of track over the loop is occupied by a large enough piece of equipment (locomotive, car, perhaps even a tie tamper, etc.), the crossing lights come on and stay on until the large object departs. Lastly there are track circuits, which are the oldest method, in which a relay drops out whenever the track circuit is occupied. Unlike track circuits, insulated joints aren't required for predictors nor presense detectors.